Geospatial Analysis

Summary

This report shows maps and time series data of potential harmful algal blooms (HABs) in several of Denver Water’s reservoirs. The reservoirs in this report were selected by Denver Water as key locations where HABs could cause increased water treatment requirements or potential public health concerns.

Map of all reservoirs in the report.

HABs may be detected through the use of remote sensing algorithms based on data from the Sentinel 2 Level 1-C Top of Atmosphere satellite. There are two key metrics shown in this report that relate to HAB presence:

NIR:Red: the near infrared (NIR) to red band ratio. The higher NIR:Red values relate to increasing algae biomass on the water surface (Tebbs et al., 2013).The NIR:Red ratio is close to zero for clear water and increases with algae concentrations and values greater than 1 indicates algae presence (Tebbs et al., 2013). NIR:Red is calculated in equation 1 below.

where R represents the top of atmosphere reflectance (0–1) for a given band.

FAI-NDWI: This algorithm uses the floating algae index (FAI) and the normalized difference water index (NDWI). This algorithm detects potential cyanobacterial blooms (Oyama et al., 2015). First, the FAI differentiates between clear water and algae using a threshold of 0.05. The FAI considers values greater than or equal to 0.05 to be algae, while those below are clear water. Next, the NDWI partitions algae (i.e., pixels with FAI ≥ 0.05) into cyanobacteria and non-cyanobacteria blooms using a threshold of 0.63. Values greater than or equal to this threshold are considered probable cyanobacterial blooms while those below are not. This algorithm is described in equations 2 and 3 below.

We applied the above algorithms to the Sentinel 2 data in Google Earth Engine to evaluate temporal and spatial patterns of algae presence and potential cyanobacterial blooms. With these data, we created a time series of the NIR:Red ratio and the FAI-NDWI cyanobacteria algorithm at each extraction point. In addition to the time series data, we also created maps for each waterbody showing the number of times each pixel exceeded a NIR:Red ratio greater than 1 and met the conditions for cyanobacteria according to the FAI-NDWI algorithm.

Dense algae on the water surface can yield taste and odor issues, additionally cyanotoxins may be present in the water with cyanobacteria presence. Therefore, remote sensing is a valuable tool to map algal blooms and potential HABs because the data are continuous, frequent, and freely available. While remote sensing cannot determine toxicity, it offers near real time results and provides drinking water managers a tool to understand the seasonality and presence of algal blooms to assist in decision making.

Interpreting Results

Detection points were placed in four different areas around each reservoir. Points are labeled edge, near edge, and open water to indicate the distance from shore. This is to help differentiate the satellite results during times of low water or high water. Clusters of points are labeled as MON, followed by a number, indicating which cluster of monitoring points they belong to. In addition to the monitoring points, some reservoirs have sites labeled as Chlorophyll sampling points, where previous in situ sampling of Chlorophyll-a has occurred. Maps of each studied reservoir will contain data between 2022-04-01 and 2022-06-07. The figure below shows an example of a portion of reservoir with an active algal bloom occurring, according to the NIR:Red ratios.

All three points in the monitoring cluster (MON-04) are in blue or dark blue bands. According to the legend, the “open water” monitoring point is in a location where NIR:Red ratio threshold has been exceeded 17-22 times within the time period. It is important to note that these remote sensing approaches are subject to important assumptions and limitations, such as waterbody size, mixed land-water pixels, and atmospheric effects (cloud cover, haze). For instance, raster data and the associated color bands located outside of the black waterbody outline may be picking up land pixels. Therefore, using and understanding both the maps and the time series data for each point can help determine whether harmful blooms are taking place in each reservoir.

Key Results

Table XX below describes results at each reservoir using the algorithms above. This table highlights data from the period XX to YY. The other tables and figures in this report show results for the entire season so far.

TABLE HERE

Antero Reservoir

NIR:Red

  • yes According to the NIR:Red remote sensing data, the reservoir expressed evidence of algae growth between 2022-04-01 and 2022-06-07. For each location and pixel type, the NIR:Red ratio exceeded 1 for at least several satellite overpasses.

  • no According to the NIR:Red remote sensing data, the reservoir did not express evidence of algae growth between 2022-04-01 and 2022-06-07.

FAI:NDWI

  • yes The FAI-NDWI algorithm showed potential blue-green algae growth.

  • no The FAI-NDWI algorithm did not show potential blue-green algae growth.

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The following images are maps showing potential harmful algal blooms in Antero Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Antero Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Bambei-Walker Reservoir

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The following images are maps showing potential harmful algal blooms in Bambei-Walker Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Bambei-Walker Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Chatfield Reservoir

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The following images are maps showing potential harmful algal blooms in Chatfield Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Chatfield Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Cheesman Reservoir

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The following images are maps showing potential harmful algal blooms in Cheesman Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Cheesman Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Dillon Reservoir

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The following images are maps showing potential harmful algal blooms in Dillon Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Dillon Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Dunes Reservoir

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The following images are maps showing potential harmful algal blooms in Dunes Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Dunes Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Gross Reservoir

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The following images are maps showing potential harmful algal blooms in Gross Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Gross Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Marston Reservoir

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The following images are maps showing potential harmful algal blooms in Marston Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Marston Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Platte Canyon Reservoir

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The following images are maps showing potential harmful algal blooms in Platte Canyon Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Platte Canyon Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Ralston Reservoir

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The following images are maps showing potential harmful algal blooms in Ralston Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Ralston Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Strontia Springs Reservoir

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The following images are maps showing potential harmful algal blooms in Strontia Springs Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Strontia Springs Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Tanabe Reservoir

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The following images are maps showing potential harmful algal blooms in Tanabe Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Tanabe Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Welby Reservoir

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The following images are maps showing potential harmful algal blooms in Welby Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Welby Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

Williams Fork Reservoir

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The following images are maps showing potential harmful algal blooms in Williams Fork Reservoir from 2022-04-01 to 2022-06-07.

NIR:Red mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher NIR:Red exceedances. MON in the site name means monitor.

FAI-NDWI mapped from 2022-04-01 to 2022-06-07 study period. Darker colors indicate higher FAI-NDWI exceedances.

At Williams Fork Reservoir, timeseries data is processed for at least four locations as shown in the figure below. At each monitoring point, three different timeseries are collected at three sites spaced 10m apart - edge, near edge, and open water. “MON” in the site name means monitor, and the additional point location names correspond to sampling point names previously used at Denver Water .

NIR:Red time series from 2022-04-01 to 2022-06-07 study period. The blue dashed line is at NIR:Red = 1.”

References

Oyama, Y., Matsushita, B., & Fukushima, T. (2015). Distinguishing surface cyanobacterial blooms and aquatic macrophytes using Landsat/TM and ETM+ shortwave infrared bands. Remote Sensing of Environment, 157, 35–47. https://doi.org/10.1016/j.rse.2014.04.031

Tebbs, E. J., Remedios, J. J., & Harper, D. M. (2013). Remote sensing of chlorophyll-a as a measure of cyanobacterial biomass in Lake Bogoria, a hypertrophic, saline–alkaline, flamingo lake, using Landsat ETM+. Remote Sensing of Environment, 135, 92–106.